A Microsoft file hosting and synchronization service.
Open Word.
Go to File - Options.
Click Save on the left side of the Options dialog.
Then on the right, you can set the Office apps to disable autosave to OneDrive and set the Office apps to save to your PC by default.
If you have OneDrive enabled to sync your files on your PC, that will automatically sync the Desktop, Documents and Pictures folders, so you would need to save your Office files to a different folder that is not synced to OneDrive.